[PATCH v11] nptl: fix potential merge of __rseq_* relro symbols

[Florian Weimer]

* Michael Jeanson:

> While working on a patch to add support for the extensible rseq ABI, we
> came across an issue where a new 'const' variable would be merged with
> the existing '__rseq_size' variable. We tracked this to the use of
> '-fmerge-all-constants' which allows the compiler to merge identical
> constant variables. This means that all 'const' variables in a compile
> unit that are of the same size and are initialized to the same value can
> be merged.
>
> In this specific case, on 32 bit systems 'unsigned int' and 'ptrdiff_t'
> are both 4 bytes and initialized to 0 which should trigger the merge.
> However for reasons we haven't delved into when the attribute 'section
> (".data.rel.ro")' is added to the mix, only variables of the same exact
> types are merged. As far as we know this behavior is not specified
> anywhere and could change with a new compiler version, hence this patch.
>
> Move the definitions of these variables into an assembler file and add
> hidden writable aliases for internal use. This has the added bonus of
> removing the asm workaround to set the values on rseq registration.
>
> Tested on Debian 12 with GCC 12.2.
>
> Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Florian Weimer <fweimer@redhat.com>
> Cc: DJ Delorie <dj@redhat.com>

This version looks good to me.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

I'll push it for you.

Thanks,
Florian